METHOD AND APPARATUS FOR PRODUCING A POROSITY LOG OF A SUBSURFACE FORMATION CORRECTED FOR DETECTOR STANDOFF

摘要

A borehole logging tool is lowered into a borehole traversing asubsurface formation and a neutron detector measures the die-away ofnuclear radiation in the formation. A model of the die-away ofnuclear radiation is produced using exponential terms varying as thesum of borehole, formation and thermal neutron background components.Exponentially weighted moments of both the die-away measurements anddie-away model are determined and equated. The equated moments aresolved for the ratio of the borehole to formation amplitude componentsof the measurements. The formation decay constant is determined fromat least the formation and thermal neutron background terms of theweighted measurement aril model moments. An epithermal neutronlifetime is determined from the formation decay constant. Thisepithermal neutron lifetime and the amplitude ratio are used by atrained neural network to determine a lifetime correction and anapparent standoff. A standoff corrected lifetime is determined fromthe epithermal neutron lifetime and the lifetime correction. Aporosity log of the formation is produced which is corrected fordetector standoff from the borehole wall as a function of the standoffcorrected epithermal neutron lifetime calibrated in borehole models ofknown porosities.